A switched-mode power converter (also referred to as a “power converter”) is an electronic power processing circuit that converts an input voltage waveform into an output voltage waveform. The waveforms are typically, but not necessarily, dc waveforms, generated by periodically switching power switches or switches coupled to an inductive circuit element. The switches are generally controlled with a conduction period “D” referred to as a “duty cycle.” The duty cycle is a ratio represented by the conduction period of a switch to a switching period thereof. Thus, if a switch conducts for half of the switching period, the duty cycle for the power switch would be 0.5 (or 50 percent).
Feedback controllers associated with power converters manage an operation thereof by controlling the conduction period of a switch employed therein. Generally, a feedback controller is coupled to an output of a power converter in a feedback loop configuration (also referred to as a “control loop” or “closed control loop”) to regulate an output characteristic of the power converter such as an output voltage. A switched-mode power converter typically receives a dc input voltage Vin from a source of electrical power at input nodes thereof and provides a regulated output voltage Vout at output nodes thereof to power, for instance, a microprocessor coupled to the output nodes of the power converter.
Switched-mode power converters are key components in many commercial and military systems for the conversion, control and conditioning of electrical power, and often govern the performance and size of the end system. Power density, efficiency and reliability are key metrics used to evaluate power converters. Magnetic devices including isolation transformers (also referred to as “transformers”) and inductors used within the power converters contribute a significant percentage to the volume and weight and, hence, determine power converter power density, efficiency, and reliability.
Integrated magnetics provide a technique to combine multiple inductors and/or transformers in a single magnetic core. Integrated magnetics are specifically amenable to interleaved current multiplier power converter topologies where the input or output current is shared between multiple inductors. Integrated magnetics offer several advantages such as improved power density and reduced cost due to the elimination of separate magnetic components, reduced switching ripple in inductor currents, and higher efficiency due to reduced magnetic core and copper losses.
For applications where higher currents (typically greater than 50 amps (“A”)) are required at low (typically less than 3.3 volts (“V”)) to moderate (typically about 12 V) voltages at high efficiency and power density, a two-phase interleaved current multiplier power converter might be inadequate to meet switching ripple specifications on inductor currents and output voltage. A larger output capacitor can reduce the output ripple voltage, but will increase the volume and weight of the power converter and result in sluggish transient response to dynamic load conditions. Multiphase, interleaved current multiplier power converters beyond the present two-phase designs are required for such applications. Utilizing multiple discrete E-cores to implement multiphase interleaved current multiplier power converters and simply paralleling multiple power converters, however, increases component count and interconnect losses, resulting in poor power density and efficiency.
An additional limitation to using E-cores for high current applications is the detrimental effects of fringing flux due to the limited cross-sectional area of a gapped center leg. Fringing flux represents the flux component that strays away from the main magnetic path and spills into the core window, inducing eddy currents in the windings therein. This results in increased losses (e.g., denoted by I2R, wherein “I” represents the current and “R” represents the resistance) in the windings and reduced efficiency. To reduce the induction of eddy currents due to fringing flux, windings are placed a safe distance from the air gap, resulting in poor utilization of the core window area. In addition, fringing flux represents a loss of inductance, which results in increased switching ripple in the winding currents, leading to higher losses and poorer efficiencies.
Power converters may also often employ current-doubler rectifiers coupled to the secondary winding of a transformer. The transformer is employed typically to accommodate widely dissimilar input and output terminal voltages. The current-doubler rectifiers interleave two currents to produce an output current with double the ripple frequency. A current-doubler rectifier can produce a low output voltage at high current with reduced losses in the secondary winding of the transformer. To produce higher levels of current interleaving to improve output voltage filtering, however, multiple power converters are often coupled in parallel, which results in replication of similar parts to form the power converter topology.
Multiphase power converter topologies can thus provide highly desirable power converter designs, not only for small size, but also for the ability to provide fast response times for a controller regulating the output voltage thereof. A power converter that combines the advantages of an interleaving, multiphase power converter topology with integrated magnetics and a current multiplying rectifier is not presently available for the more severe applications that lie ahead.
Accordingly, what is needed in the art is a power converter topology that employs a switching circuit and a rectifier such as a current multiplying rectifier that can provide higher levels of interleaving without the component replication that is necessary using presently available power converter circuits such as current-doubler rectifiers.